This invention relates, in general, to radioactivity well logging, and more particularly to methods and apparatus for determining the macroscopic thermal neutron capture cross-section of the formations surrounding a borehole as determined by radiation measurements.
It is well known in the art of radioactivity well logging, for example, as illustrated and described in U.S. Pat. Nos. 3,379,882 and 3,379,884 which issued to Arthur H. Youmans and each of which is assigned to the assignee of the present invention, to measure the macroscopic thermal neutron capture cross-section [Sigma (.SIGMA.)] of the formations surrounding a borehole. This prior art method makes such a measurement or computation by measuring the decline of the thermal neutron population in such formations within a fixed period of time following the emission of a burst of high energy neutrons and by dividing the radiations indicative of such thermal neutrons into two equal groups and computing the rate of change over the selected time interval. In U.S. Pat. No. 3,566,116, use is made of two measurement intervals so that the starting time and the duration of the two measurement intervals can be continuously adjusted so as to maintain a fixed counting ratio between the two measurement intervals.
Yet another method of measuring the macroscopic thermal neutron capture cross-section of the formation is described in U.S. Pat. No. 4,046,764, which issued to Gerald L. Marquis and which is assigned to the assignee of the present invention. This method makes use of establishing the points in time of which radiation is detected within a fixed duration time interval beginning at a fixed time following each neutron burst from the source. A unique solution is obtained where each measured value of the time relationship defines a single value of macroscopic neutron absorption cross-section.